Functional Coatings for III-V Semiconductor Photo(electro-)catalysts

Over the past decade, protective coatings have been essential for light-harvesting semiconductors. It provides multiple functionalities including anti-reflection, catalysis, and photo-corrosion protection. We report advances in coating discovery and interfacial tunability for photoelectrocatalysts and photocatalysts. First, we show that precise microstructural control, in the length scale across angstroms to micrometers, is vital for device performance.[1] The challenge remains for all practical semiconductor photoabsorbers of 0.7-2.3 eV bandgaps, that leaky TiO2 protective coating cannot achieve the theoretical efficiency limit for any III-V semiconductors, e.g., Gallium Phosphide. We will discuss the rationale of discovering manganese-alloyed titanium oxides and achieving a 1 V photovoltage and 100 hour stability from GaP/(Ti,Mn)Ox/iridium-cocatalyst photoelectrocatalysts [2] by tuning microstructures using thermal processing.[3] Another challenge is the lack of stable coatings for fuel-forming photocatalysts that drive reductive reactions: TiO2 is not stable under reductive potentials, because Ti⁴⁺ ions in the coating can be easily reduced to Ti³⁺ and dissolved under cathodic conditions. We further developed a novel gallium nitride (GaN_x) coating to stabilize and passivate III-V semiconductors using atomically abrupt interfaces. During photo-electrocatalytic H2 evolution, the GaNx/p-InP photocathode exhibits a photocurrent density of 34 mA/cm2 under 1 sun illumination, and stability of at least 150 hours in pH 0 acid. Beyond efficiency and stability, we can tune the effective band edge positions of semiconductor photocatalysts via unique electronic structures and charge density. [4] We discuss the principle of using microstructures, oxidation states, and point defects as tuning knobs.<br/>[1] D. Solanki, C. He, Y. Lim, R., Yanagi, S. Hu, “Where Atomically Precise Catalysts, Optoelectronic Devices, and Quantum Information Technology Intersect: Atomic Layer Deposition of Ternary Materials”, Chemistry of Materials, 36, 3, 1013–1024 (2024).<br/>[2] X. Shen, T. Zhao, H. Su, M. Yang, J. Chen, Y. Liu, J. Li, R. Yanagi, D. Solanki, and S. Hu, “Tuning intermediate bands of protective coatings to reach the bulk-recombination limit of stable water-oxidation GaP photoanodes”, Advanced Energy Materials, 12, 220314 (2022).<br/>[3] D. Solanki, J. A. Röhr, Z. S. Fishman, B. Liu, R. Yanagi, E. Stavitski, D. Lu, S. Hu, “Probing Rutile Solid-Phase Crystallization of Atomically Mixed Mn-Alloyed TiO2 Coatings through XANES Analysis”, MRS Communications, 13 (26) (2023).<br/>[4] R. Yanagi, T. Zhao, M. Cheng, B. Liu, H. Su, C. He, J. Heinlein, S. Mukhopadhyay, H. Tan, D. Solanki, and S. Hu, “Photocatalytic CO2 Reduction with Dissolved Carbonates and Near-Zero CO2(aq) by Employing Long-Range Proton Transport”, Journal of the American Chemical Society, 145, 28, 15381-15392 (2023).